In an earlier patent application, Ser. No. 07/506,418, now U.S. Pat. No. 5,063,343, entitled "Current Pump Structure," a current matching circuit is disclosed similar to that shown in FIG. 1. This circuit of FIG. 1 consists of an output current source comprising transistor Q1 and resistor R1, a current mirror output portion comprising switchable transistors Q2 and Q3, and a current mirror programming portion comprising the circuitry within dashed-outline 12.
Basically, the current mirror of FIG. 1 operates to generate the same current through current mirror output transistors Q2 and Q3 (assuming diodes D1 and D2 are off) as is being generated through transistor Q5 by providing a same gate-source voltage to each of transistors Q2, Q3, and Q5. The current source comprising transistor Q4 and resistor R4 is used to match the output current source. In this way, ideally, the current supplied to output terminal 10 through transistors Q1 and R1 will be equal to the current sunk through either transistor Q2 or transistor Q3. By controlling diodes D1 and D2 to conduct or not conduct, a plus current, a minus current, or a zero current may be generated at output terminal 10.
This current matching circuit of FIG. 1 operates adequately for most purposes. However, for non-ideal output current sources, the current supplied to output terminal 10 will change as the voltage on terminal 10 increases. For example, when the depletion mode transistor Q1 is fabricated using Gallium Arsenide (GaAs) technology, the current flow through transistor Q1 is undesirably varied by a change in voltage at the source of transistor Q1 due to a change in voltage on output terminal 10. This effect on the current flow through transistor Q1 due to a change in voltage at the source of transistor Q1 is called backgating.
Backgating is a voltage related phenomena occurring in GaAs devices, which is similar to the body effect found in MOS devices. A negative bias on the substrate with respect to the source depletes the channel under a gate 5 further, such that a more positive potential is required on the gate to invert the channel and turn the device ON. The effect is a positive increase in the device threshold voltage V.sub.T necessary to the turn on the device.
In the depletion mode transistor Q1 of FIG. 1, serving as a current source, the drain voltage is fixed, and the gate and source nodes move with respect to ground and the substrate (tied to ground). Therefore, when the source voltage moves due to a change in voltage on output terminal 10, the current outputted by transistor Q1 will no longer match the current being sunk by either transistor Q2 or Q3.
As another example, MOS designers would experience the same problem (called body effect) if they used NMOS devices for VCC anchored current sources.
Thus, current source circuits (such as transistor Q1 and resistor R1) using GaAs or NMOS devices may undesirably produce an output current which varies with a change in an output voltage.
What is needed is a compensation circuit for a current mirror (such as the current mirror comprising transistors Q2 and Q3) which causes the current mirror to generate a current output which mirrors the voltage dependent behavior of a current load (such as the current source comprising transistor Q1 and resistor R1) which the current mirror output is intended to match.